Reactive and potentially toxic cofactors such as copper ions are imported into eukaryotic cells and incorporated into target proteins by unknown mechanisms. Atx1, a prototypical copper chaperone protein from yeast, has now been shown to act as a soluble cytoplasmic copper(l) receptor that can adopt either a two-or three-coordinate metal center in the active site. Atx1 also associated directly with the Atx1-like cytosolic domains of Ccc2, a vesicular protein defined in genetic studies as a member of the copper-trafficking pathway. The unusual structure and dynamics of Atx1 suggest a copper exchange function for this protein and related domains in the Menkes and Wilson disease proteins.